1. Field of the Invention
The present invention relates to the field of image processing, and more particularly, to a method and apparatus for color transformation with minimized transformation errors.
2. Description of the Related Art
As electronic technology advances, information provided to users includes not only simple text, but also various multimedia information such as snapshots, motion pictures, animation, and sounds. In particular, motion pictures are widely studied since next generation video on demand (VOD) services and interactive services are based on them.
Thanks to the development of digital electronics, conventional analog signals can be converted to digital data and techniques for processing various types of digital video media to manage vast amounts of data efficiently have been introduced. Some of the merits of digital image processing technology are as follows.
First, every analog device is subject to noise during processing operation such as signal transmission and restoration. Thus, the resolution of images is very likely to be degraded while performing restoration of recorded image signals. However, digital image processing devices are resistant to such noise.
Second, it is possible to process analog signals with computers by digitalizing them. Various processing methods such as compression are realized by processing image signals with computers.
Digital image processing techniques mainly concern how to process analog signals recorded on a medium by a computer. Digital image processing techniques are realized by using a Digital Video Interactive (DVI) method. The DVI method enables a processor adapted to perform instructions suitable for processing images to perform functions that cannot easily be executed by normal processors in a short period of time.
Furthermore, two expert groups, the Joint Photographic Experts Group (JPEG) and the Motion Picture Experts Group (MPEG), have promulgated a coding standard with DVI capability, and this coding standard is expected to play a significant role in digital image processing techniques since most companies are supporting it. In particular, the MPEG standard is not only used for processing images on personal computers, but it is also widely used for high definition systems such as High Definition Televisions (HDTVs). Subsequent updates to the MPEG standard such as MPEG II and MPEG III have been accomplished.
Since 1991, techniques for processing images by only using the processing capacity of main processors without requiring specialized hardware has been introduced, and QuickTime of Apple company and Video for Windows of Microsoft and Indeo of Intel are commonly used. These image processing techniques are especially suitable to personal computers thanks to advancing high speed main processors.
With various digital image processing techniques introduced, attempts to standardize various techniques have been made. Standardized digital image processing techniques are not only used for video conference systems, digital broadcasting codec systems, and video telephone systems, but they are also shared and supported in computer industries and communication industries. For example, digital compression techniques for storing information on optical disks such as CD-ROMs or digital recording media is realized by a technique which is very similar to a compression technique for video conferencing. Nowadays, MPEG standardization is being made by ISO-IEC, JTC1, SC1, and WGI 1.
For efficient use of digital image processing techniques, a preprocessing operation which transforms color signals in RGB color space to other color spaces is required. That is, color space transformation, filtering, and color subsampling are performed in the preprocessing operation.
Color space transformation means transforming color images made of R, G, B components into components representing luminance Y and chrominance of the image. Information of R, G, and B color signals overlaps, but most information including delicate regions of an image is mapped into luminance information Y while redundant color information is left in the chrominance information by using the color space transformation. This is because human eyes are more sensitive to luminance variations than to the chrominance variations.
FIG. 1 depicts a color transformation technique according to the conventional art.
In FIG. 1, the RGB color space is transformed into a YCbCr color space by using equation 1.
                                          [                                                            Y                                                                              Cb                                                                              Cr                                                      ]                    =                                    Φ              YCbCr                        ⁡                          [                                                                    R                                                                                        G                                                                                        B                                                              ]                                      ,                                  ⁢                              [                                                            Y                                                                              Cb                                                                              Cr                                                      ]                    =                                    [                                                                    0.2126                                                        0.7152                                                        0.0722                                                                                                              -                      0.1146                                                                                                  -                      0.3854                                                                            0.5                                                                                        0.5                                                                              -                      0.4542                                                                                                  -                      0.0458                                                                                  ]                        ⁡                          [                                                                    R                                                                                        G                                                                                        B                                                              ]                                                          (        1        )            
The inverse transformation is performed by equation 2.
                                          [                                                            R                                                                              G                                                                              B                                                      ]                    =                                    Φ              YCbCr                              -                1                                      ⁡                          [                                                                    Y                                                                                        Cb                                                                                        Cr                                                              ]                                      ,                                  ⁢                              [                                                            R                                                                              G                                                                              B                                                      ]                    =                                    [                                                                    1.0                                                        0.0                                                        1.5748                                                                                        1.0                                                                              -                      0.1873                                                                                                  -                      0.4681                                                                                                            1.0                                                        1.8556                                                        0.0                                                              ]                        ⁡                          [                                                                    Y                                                                                        Cb                                                                                        Cr                                                              ]                                                          (        2        )            
The color transformation techniques of the prior art are only restricted to maintaining compatibility with black and white signal processing. However, it has been changed in order to transmit high quality, and high resolution image information at high speed. The H.264/AVC standard includes techniques that are mainly aimed at reducing color transformation errors.
A color transformation method invented by Parchem, et al. and filed by Microsoft in U.S. patent application Ser. No. 5,745,119 is depicted in FIG. 2.
FIG. 2 shows a transformation operation converting color signals in the RGB color space into a YCoCg color space.
As shown in FIG. 2, the RGB color space is transformed into a YCoCg color space using a transformation function (ΦYCoCg). The YCoCg signals are digitally-processed, and the processed signals are inverse-transformed into the RGB color space using an inverse transformation function (ΦYCoCg−1).
As shown in FIG. 2, the transformation to the YCoCg color space is performed using equation 3.
                                          [                                                            Y                                                                              Co                                                                              Cg                                                      ]                    =                                    Φ              YCoCg                        ⁡                          [                                                                    R                                                                                        G                                                                                        B                                                              ]                                      ,                                  ⁢                              [                                                            Y                                                                              Co                                                                              Cg                                                      ]                    =                                    [                                                                                          1                      /                      4                                                                                                  1                      /                      2                                                                                                  1                      /                      4                                                                                                            1                                                        0                                                                              -                      1                                                                                                                                                          -                        1                                            /                      2                                                                            1                                                                                                      -                        1                                            /                      2                                                                                  ]                        ⁡                          [                                                                    R                                                                                        G                                                                                        B                                                              ]                                                          (        3        )            
The inverse transformation to the RGB color space is performed by using equation 4.
                                          [                                                            R                                                                              G                                                                              B                                                      ]                    =                                    Φ              YCoCg                              -                1                                      ⁡                          [                                                                    Y                                                                                        Co                                                                                        Cg                                                              ]                                      ,                                  ⁢                              [                                                            R                                                                              G                                                                              B                                                      ]                    =                                    [                                                                    1                                                                              1                      /                      2                                                                                                                          -                        1                                            /                      2                                                                                                            1                                                        0                                                                              1                      /                      2                                                                                                            1                                                                                                      -                        1                                            /                      2                                                                                                                          -                        1                                            /                      2                                                                                  ]                        ⁡                          [                                                                    Y                                                                                        Co                                                                                        Cg                                                              ]                                                          (        4        )            
Similarly to FIG. 2, U.S. patent application Ser. No. 6,421,464 invented by Tran et, al. and filed by FastVDO introduces a transformation method for transforming an RGB color space to a YFbFr color space. The color transformation techniques introduced by both Microsoft and FastVDO guarantee reversibility using an integer mapping and lifting scheme.
However, these color transformation techniques are based on ideal processing operation, and coding errors are inevitable in real operation. Coding errors occur during inverse transformations as well as during forward transformations.
Thus, color transformation techniques with reduced coding errors are highly required.